The present invention relates to an apparatus and method for controlling a hybrid car having an engine and electric motors.
In a hybrid car using an engine and electric motors, a power generated by the engine is converted to an electric energy directly or through a generator, while the electric energy is converted to a mechanical energy by a motor or is stored in a battery. Such a hybrid car is advantageous in that the running distance can be made long in comparison with an electric car in which the supply of energy is done with a battery alone.
In Japanese Patent Laid-Open No. Hei 9-37410 there is disclosed a configuration provided with an engine, a power distributing mechanism, and motor generators.
In such a hybrid car, when the required driving torque is large, the car is driven by the engine, part of the engine power is distributed to a motor generator which is for assisting the vehicle speed, allowing the motor generator to act as a generator, then with the generated power from the generator, the torque from a driving motor is assisted to increase the driving torque.
On the other hand, when the required driving torque is small and the vehicle speed is high, part of the engine driving torque is recovered from a motor generator which is for assisting the driving torque, and with this electric power, a differential motor generator is allowed to operate as a motor, thereby permitting a vehicular operation at a high speed.
In such a configuration, when the car is to be driven at a low speed and at a small driving torque required (a small driving output required), it is necessary that the vehicular operation be done in a region of a small engine torque. In such a small engine torque region, there is a tendency to an increase of pumping loss and deterioration of fuel economy because the vehicular operation is performed in a closed state of the throttle valve. If the operation in a large pumping loss region is restricted, there arises the problem that the engine operating region becomes small and the motor size increases to assist torque for acceleration. If the engine operation is topped in low speed and low torque conditions, the operational proportion using the battery increases, thus requiring a larger battery capacity or more frequent charge/discharge control for the battery. A highly efficient operation can be realized by combining the engine with a transmission, selecting a shift gear (a change gear ratio in case of a stepless change gear ratio) and performing operation in a region where the engine torque is as high as possible. However, there arises the problem that, since the operating torque has already approached its maximum level, there remains no marginal torque in acceleration, thus resulting in a poor accelerative feeling and deterioration of the driving performance.
Accordingly, it is the first object of the present invention to provide a hybrid car having an engine and electric motors which car can effect a highly efficient operation without increasing the motor and battery capacities.
It is the second object of the present invention to ensure a superior driving performance in a highly efficient operation.
The above first object of the present invention can be achieved by allowing a lean burn to take place to enlarge the region of the highly efficient operation and increasing the proportion of operation with the engine at a low torque while decreasing the proportion of motor operation using the battery. Lean burn is advantageous in that the pumping loss can be diminished because the throttle valve is opened. The pumping loss of the engine at a low torque may be diminished by controlling the intake valve timing to control the amount of intake air.
The above second object can be achieved by selecting a region of a large number of revolutions of the engine to ensure a marginal torque for operation at a required engine output. For example, whether the driver of the car attaches importance to fuel economy or to the driving performance is judged in accordance with a change in the degree of opening of an accelerator pedal. If the driver attaches importance to the driving performance, there is selected an engine operation region of a large marginal torque. In the high engine speed region, driving performance takes precedence over other points although fuel economy becomes worse than at the highest efficiency point.